Modern wireless communication systems employing wideband Power Amplifiers (PAs) must be operated close to saturation to maximize power efficiency. This introduces significant non-linear signal distortion to wideband signals such as Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) signals due to their large peak-to-average power ratio characteristics. Techniques to mitigate this non-linear distortion include:                Backing off the PA operating point. However, doing so leads to larger size and cost to yield the same output power.        Feed-forward cancellation of the non-linear distortion with analog combining networks. This is effective but incurs the additional cost of feed forward cancellation circuits and requires an additional low-power but highly linear Radio Frequency (RF) amplifier.        Baseband Digital Predistortion (BB-DPD), which employs Digital Signal Processing (DSP) techniques to impress an “inverse characteristic” of the PA on the transmitted signal to compensate for the non-linear distortion introduced by the PA.BB-DPD has become by far the preferred approach for managing PA nonlinearities.Utilizing adaptive BB-DPD to compensate for the non-linearity of the PA is a proven technology that enables high linearity, high efficiency power PA sub-systems for single-band transmitters. However, in applications such as base stations of a cellular communications network that support multiple radio access technologies or multiple bands for the same radio access technology, multi-band transmitters are desirable. Conventional BB-DPD systems are not optimal for multi-band transmitters.        
In order to address the complexity and power consumption of digital BB-DPD, RF Analog Predistortion (RF-APD) for single-band signals has been proposed by entities such as Scientera. RF-APD technology predistorts for non-linear distortion using analog delay cells and Gilbert cell multipliers. It is difficult to design these circuit elements with sufficiently wide bandwidth to address multi-band signals, particularly those with a wide separation between individual bands. An academic example of RF-APD, Yi et. al., “Analog Predistortion Linearizer for High-Power RF Amplifiers,” IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 12, December 2000, illustrates the technical challenge of implementing RF-APD for a single-band signal of only 30 megahertz (MHz) bandwidth. Finally, analog circuits suffer from thermal, voltage, and semiconductor process variations that must be compensated for using sophisticated on-line calibration circuitry.
In addition, an academic example of digital predistortion at RF for a single-band signal is described in Mark Sterling et al., “Direct Digital Predistortion on a Computer Controlled FPGA,” IEEE International Conference on Acoustics, Speech and Signal Processing, 2007, Vol. 2, pp. II-369, II-372, Apr. 15-20, 2007. Here, the real-valued digital RF signal is predistorted by passing it through the following digital operations: a cubic non-linearity (implemented as a lookup table) followed by a gain adjustment followed by a time delay. This method makes no attempt to manage the digital aliasing of third order terms generated from cubing the single-band signal. Managing this aliasing becomes infeasible for multi-band signals whose third order distortion products can alias into signal bands of interest within the active Nyquist zone.
As such, there is a need for systems and methods for digital predistortion in a multi-band transmitter.